Magnetic tracking system

ABSTRACT

The present invention provides an electromagnetic tracking system that includes a field generator and a field sensor arranged to generate and detect, respectively, an electromagnetic field. Both the transmitter and receiver coils are connected to signal conditioning and processing circuitry to provide outputs indicative of the coil signals. A processor operates on the signals to determine the coordinates of the sensing assembly relative to the generator assembly. The signal processor produces ratiometric outputs, and applies a mutual inductance model to solve for position/orientation coordinates. In some embodiments, a disturber in the form of a conductive ring or a sheath is disposed about an interfering piece of equipment to moderate and standardize disturbances due to eddy currents.

BACKGROUND OF THE INVENTION

The present invention relates to magnetic tracking systems of the typewherein a magnetic field is established in a relevant work area, and oneor more magnetic field sensors are operated to sense values of a localmagnetic field and are processed to determine the position of a tool,instrument or other identified object. In general, such systems operateusing a field generating element or assembly, and a field sensingelement or assembly, often in conjunction with other positiondetermining elements, such as robotic stepper or optical trackerassemblies to track the relative changes in position between one or morefixed points or structures in the physical environment or associatedimages, and one or more moving or non-visible points or structures inthe work arena.

Magnetic field generating or sensing assemblies for tracking may beimplemented in various ways, with conventional analog wire coils formingcurrent loops or paths, or with semiconductor ormicrolithographically-formed conductive leads or circuit board tracesforming current paths, arranged in an appropriate geometry to generateor sense the desired field components. There may be a symmetry orduality between the generating or sensing elements. Thus, for example inmany cases it is possible to have a small multi-coil array thatgenerates a spatially distributed magnetic field and a similar or evenidentical array that senses the field so generated. Small coils offerthe prospect of generating, to a close approximation, dipole fields,although small size may limit the attainable field strength or theachievable level of detection signal amplitude. The generating andsensing constructions may alternatively employ different scales, forexample, with relatively large and/or high current coils to establishmagnetic field components along different axes, and smaller, or morelocalized coil assemblies for sensing field values. Smaller coils,whether for sensing or generating, may, for example, be fastened to thebody, or attached to workplace or surgical instruments, or to cathetersor other body-inserted devices, to sense the magnetic field and trackposition of the attached structure.

In general, it is the aim of such magnetic tracking assemblies to definethe spatial coordinates (e.g., position and orientation coordinates,either absolute or relative) where the movable magnetic assembly islocated at a given instant in time. It is therefore necessary tocharacterize the magnetic field distribution or signal values with somedegree of accuracy, and also necessary to accurately detect the field.The field distribution may be determined by a combination of fieldmodeling and empirical field mapping. The latter, for example, may becarried out as a calibration or an initialization step, and may beperformed to correct a theoretical field distribution for the presenceof interfering structures. In any case, the spatial coordinates aregenerally computed for one magnetic assembly (transmitter or sensor)with respect to the other magnetic assembly (sensor or transmitter).Typically, one of these assemblies is itself fixed.

One area in which magnetic tracking has been useful is the area of imageguided surgery. Typical image guided surgical systems acquire a set ofimages of an operative region of a patient's body, and track a surgicaltool or instrument in relation to one or more sets of coordinates, e.g.,spatial coordinates of the surgical work arena, the coordinates of theimages themselves, or a target feature of the patient's anatomy. At thepresent time, such systems have been developed or proposed for a numberof surgical procedures such as brain surgery and arthroscopic procedureson the knee, wrist, shoulder or spine, as well as certain types ofangiography, cardiac or other interventional radiological procedures andbiopsies. Such procedures may also involve pre-operative orintraoperative x-ray images being taken to correct the position of andrefine the display of, or otherwise navigate a tool or instrumentinvolved in the procedure in relation to anatomical features ofinterest.

Several areas of surgery have required very precise planning andcontrol. Such tracking is useful for the placement of an elongatedprobe, radiation needle, fastener or other article in tissue or bonethat is internal or is otherwise positioned so that it is difficult toview directly. For brain surgery, stereotactic frames may be used todefine the entry point, probe angle and probe depth to access a site inthe brain. Furthermore, many of the foregoing techniques may be used inconjunction with previously-compiled three-dimensional diagnostic imagessuch as MRI, PET or CT scan images to provide accurate tissuecoordinates to which the tracked physical elements may be referenced.Such systems offer valuable improvements for procedures such asplacement of pedicle screws in the spine, where visual and fluoroscopicimaging directions cannot capture the axial view that would be requiredto safely orient the insertion path through bony material close to thespinal cord.

When used with existing CT, PET or MRI image sets, the previouslyrecorded diagnostic image sets, by virtue of their controlled scanformation and the spatial mathematics of their reconstructionalgorithms, define a high precision three dimensional rectilinearcoordinate system. However, even when provided with such referenceimages it is necessary to correlate and fit available measurement andviews and anatomical features visible from the surface, with features inthe diagnostic images and with the external coordinates of the toolsbeing employed. This is often done by providing implanted fiducials, byadding external visible or trackable markers that may be imaged, and byenabling a surgeon or radiologist to use a keyboard or mouse to identifyfiducials or features in the various images, and thus map common sets ofcoordinate registration points in the different images. Given a fit ofspatial points to image points, software may then track changingpositions in an automated way (for example, simply transforming thecoordinates that are output by an external coordinate measurementdevice, such as a suitably programmed off-the-shelf optical trackingassembly.) Instead of correlating image positions of a set of imageablefiducials appearing in fluoroscopic or CT images, such systems can alsooperate with simple optical tracking, employing an initializationprotocol wherein the surgeon touches or points at a number of bonyprominences or other recognizable anatomic features in order to definethe external coordinates in relation to the patient anatomy and toinitiate software tracking of those features.

For such applications, electromagnetic tracking offers the advantagethat its position-defining field, a magnetic field, penetrates the bodywithout attenuation or change so that tracking may continue during asurgical procedure, unimpaired by the blocking that occurs with visiblelight trackers (e.g., due to operating room personnel moving intopositions that obstruct the line-of-sight paths required by opticaltrackers). Optical or ultrasonic tracking, by contrast, may require alarger or excess number of line-of-sight paths, and correspondingtransponders and/or detectors to assure that triangulation is alwayspossible despite occluded pathways. The body-penetrating electromagneticfields also allow one to track locations or movements inside the bodywith minimal resort to the fluoroscopic or ultrasound techniquesnormally required for visualization.

Among electromagnetic tracking techniques, several principal approachesare known. In one, relatively large Helmholtz coils establish welldefined and highly uniform independent magnetic field components orgradients along each of the X, Y and Z axes in a work arena, and thestatic field components are detected by a localized detector todetermine position coordinates. This approach has been proposed, forexample, for cranial surgery, where such coils may define a suitablylocalized region encompassing the entire operative arena. Anotherprincipal approach involves using time-varying dipole fields e.g.,dipole fields established by driving field-generating coils with an ACcurrent signal. While the latter approach offers some processingadvantages (such as being able to synchronously demodulate inducedsignals, and thus cumulate detected signal values to enhancesensitivity, and also the ability to establish the X, Y and Z fieldcomponents at different frequencies so that detected sensor outputsignals may be separated or even demodulated simultaneously), it has thedisadvantage that varying magnetic fields induce eddy currents inconductive structures found within the field. Induced currentsthemselves then generate secondary magnetic fields, thus introducingdistortions into the expected or calibrated field distribution.Conductive or metal structures are in fact commonly present in atracking environment, whether it be an avionics, surgical or industrialtracking environment.

The latter problem has historically been addressed by the observationthat for a fixed metal disturber located at least twice as far from thefield generator as is the field sensor, the induced disturbance will below, e.g., under one percent, so that by restricting the tracked arenato a region sufficiently removed from the disturbing structure, accuracymay be achieved. However, this approach may be unrealisticallyrestrictive for certain applications, including some image guidedsurgery applications, where highly disturbing equipment (such as theimaging assembly of a fluoroscope) is necessarily placed as close aspossible to the work arena in which tracking is to occur. Another commonalternative approach would be to map the disturbed fields or detectedsignal levels that occur close to the distorters present in the workarena, so that a processor can more accurately determine coordinatesfrom the run time field values or the induced signals detected by asensor. However, as a practical matter, such mapping is not only likelyto require a time-consuming preliminary set-up operation, but mayrequire that the set-up be carried out afresh for each new arrangementof operating room equipment that introduces different distortions, i.e.,when equipment is changed or moved.

In addition to the practical problem of accurately detecting the field,there is the theoretical problem of converting the signal measurementsinto position and orientation coordinates.

On a fundamental level, the task that must be addressed by anyelectromagnetic tracking system is to computationally determine a uniqueposition from the various measured parameters (typically inducedvoltages indicative of field strength). Often the relevant equationshave several solutions, and care must be taken to operate within asingle-valued domain (typically a hemisphere, quadrant or octant), thuslimiting the selection of the initial generator or receiver fixedlocations to establish a sufficiently well behaved field region, andrestricting the allowable work arena in relation thereto, or elseproviding additional, or extrinsic data inputs to resolve ambiguities orrefine computations. Beyond encountering multi-valued coordinatesolutions, the accuracy of the coordinate determination processing maydepend quite critically on the relative positions of generating andsensing elements. Processing equations may break down or solutions maybecome poorly defined as the sensing or generating element approaches orcrosses a particular plane or axis, or one of its pitch, yaw or rollangle coordinates approaches 0 or 90 degrees, becomes too acute, orbecomes too oblique.

Another practical problem stems from the dynamic range of the sensedsignals, which varies greatly with distance/position, and may drop toquite low levels with increasing distance from the transmitter, or withdecreasing size of the generating or sensing coils. Often, as a fieldunit moves to different positions, different gains must be determined onthe fly, or additional gain stages must be used in order to obtainadequate signal values. This introduces some complexity and potentialfor error in normalizing the signals to accurately fit together readingsfrom two different or even closely contiguous regions.

Thus, while magnetic tracking offers certain significant advantages,particularly for surgical applications, there remains a need forimproved systems and processing to enhance accuracy of coordinatedeterminations.

Accordingly, it would be desirable to provide an electromagnetic trackerof enhanced accuracy.

It would also be desirable to provide an electromagnetic tracker havingenhanced immunity to common field distortions.

SUMMARY OF THE INVENTION

One or more of these and other desirable features are obtained in anelectromagnetic tracking system wherein a field generating unit and afield sensing unit are arranged to generate and to sense, respectivelyan electromagnetic field in an arena of interest. At least one of theunits is movable, and the units are connected to signal conditioning andprocessing circuitry that detects the levels of the transmitter drivesignals and the received signals, ratiometrically combining them to forma matrix representative of the mutual inductance of each of the pairs ofcomponent coils. The mutual inductance information, providing functionsof the relative positions and orientations of the two units, is thenprocessed to determine corresponding coordinates.

Since mutual inductance is a symmetric property, i.e., depending simplyon the geometries of the transmission and receiving coils and theirrelative positions, the system may interchange transmitter and receiverunits, employing quite small coil assemblies, yet use a model that givesa direct computation of coordinates without excessive iterativeapproximations, and without resort to the multitude of gain level andother normalization corrections otherwise typically needed when workingfrom magnetic field intensity measurements. The equations may beprocessed and solved in real time to effect surgical or other positiontracking, and various corrections and calibration of signal magnitude(e.g., for cross-coupling of coils or for finite-size and non-circularcoil geometries) are readily implemented.

In accordance with another aspect of the invention, the magnetictracking systems of the invention may employ signal conditioning orprocessing electronics in which relevant signals pass through fixed gainamplifiers to high precision (e.g, 24 bit A/D) digital converters. Thisbit size encompasses a dynamic range effective for multi-bitrepresentation of signals encountered at all regions of the intendedtracking area, thereby avoiding the need for variable gain or AGCelements, or for multiple or different preamps having different gainlevels, as one coil assembly moves further from the other. Thus, theprovision of a common preamplifier with a high precision samplereliminates patching and the errors due to inaccurate gain ratios betweengain states.

In accordance with another or further aspect of the invention, theprocessor employs a model or set of equations that operate, with thesensor output values, to determine the position/orientation coordinatesof the transmitter/receiver assemblies, and when a tracked magnetic coilassembly element approaches a singular region, i.e., a plane or regionwhere the model becomes ill-defined, inaccurate or lacks a solution inthe coordinate system, the processor operates by transforming to acoordinate representation in which the detected values lie in awell-defined or non-singular region. The processor then solves todetermine a coordinate measurement, and transforms back to theunderlying or original coordinate system.

In accordance with another aspect, an operating room system of theinvention may employ a conductive shield, or a sheath structureconfigured to fit about or contain an interfering component or piece ofequipment. The sheath standardizes the magnetic field disturbanceintroduced by the component. In some instances the sheath may be a metalcylinder, dimensioned to enclose the disturbing piece of equipment. Thesheath or cylinder may be formed of sheet metal of a gauge such thateddy currents are induced by the magnetic field, thus giving rise to astandardized field disturbance originating at a contour or surfaceexternal to the equipment. Alternatively, other suitable conductivematerials, such as a carbon fiber composite material, may be used. Thesheath also moderates or effectively nulls electromagnetic disturbancesoriginating within its contour, e.g., at the underlying piece ofequipment disposed within the sheath. The inclusion of this standardizeddisturbance in a system of the invention permits the interchange ofdifferent pieces of equipment without introducing excessive variationsin resulting local disturbance characteristics. Disturbances may thus bemapped, or even modeled, in a single initial set up operation, withoutnecessitating compilation of a new disturbance map for each new piece ofequipment

In accordance with another or additional aspect of the invention, ratherthan simply introducing a sheath to form a standardized disturbance, theprocessor may model such a disturbance. For example, the processor maymodel a conductive sheath fitted about a certain region as a conductivering or cylinder at that region (using the known dimensions and behaviorcharacteristics of the sheet metal material). The modeled disturbancemay then be added to the stored values of a map of the undisturbedelectromagnetic field to form an enhanced field map, or may be otherwiseapplied to enhance accuracy of tracking determinations. The modeledfield may also be used to provide a seed value for determining positionand orientation coordinates. A fitting procedure then refines theinitial value to enhance the accuracy of the P&O determination.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will be understood from thedescription below and accompanying claims, taken together with theFigures illustrating embodiments thereof wherein:

FIG. 1 illustrates basic hardware elements of a tracking system inaccordance with the present invention;

FIG. 2 is a hardware block diagram of a preferred embodiment;

FIG. 3 is a high level flow chart illustrating operation of a systemaccording to one aspect of the invention;

FIG. 4 illustrates signal processing for determining coordinates withthe system of FIG. 3;

FIGS. 5A-5F illustrate circuit details of some exemplary embodiments;

FIG. 6 illustrates aspects of distortion correction in accordance withother aspects of the invention;

FIG. 7 is a network model of the system of FIG. 6;

FIG. 8 illustrates geometry of fields for the configuration of FIG. 7;

DETAILED DESCRIPTION

The present invention is an improved electromagnetic tracking systemwhich may, by way of example, be employed in an operating room orclinical setting to determine the position and orientation of an objectsuch as a catheter, surgical tool, marker or the like in relation toother physical structures, or in relation to images that may, forexample, have coordinates assigned thereto by other means, such as viacoordinatized 3-D image sets, position-defining frames, or correlationwith imaged or tracked fiducials or markers. A great number of suchsystems are known, being described for example in U.S. Pat. No.5,967,980 and related patents of M. Ferre et al, U.S. Pat. No.5,307,072, and others, so the features of such systems will not befurther described here.

As relevant to a first aspect of the present invention, anelectromagnetic tracking system includes a magnetic field generator anda magnetic field sensor together with signal conditioning circuitry anda processor for processing the conditioned signals. The generator andsensor are also referred to as transmitter and receiver below. Ingeneral, a varying current injected into one coil assembly produces amagnetic field, and this field induces a voltage in the other coilassembly. The induced voltage V is proportional to the rate of change ofthe applied current I, with a constant of proportionality equal toL_(m), the mutual inductance. According to one aspect of the invention,a magnetic tracking system is configured to apply the transmitter drivesignals and the induced receiver assembly signals to determine mutualinductances, from which the processor then determines thecoordinates—i.e., the position and orientation coordinates, or simply“P&O”—of one coil assembly of the system with respect to the other coilassembly.

For two closed circuits the mutual inductance is given by Neumann'sformula, and is equal to an expression which depends only on thegeometry of the closed circuits (e.g., the coils), and not on theelectronics used to measure the mutual inductance. Since the mutualinductance L_(m) is a ratio independent of applied current, wave form orfrequency, it remains a well defined measurement parameter under a widevariety of conditions, offering improved performance in real operatingenvironments. With three transmitting coils and three receiving coils,nine mutual inductances provide an over-determined system for derivationof the position and orientation coordinates.

In some embodiments, the roles of transmitter and receiver assembliesmay be interchanged, each comprising a substantially identical threeaxis dipole coil assembly. Preferably these assemblies are built so thethree coils of an assembly each exhibit the same effective area, areoriented orthogonally to one another, and are centered at the samepoint. For many of the applications alluded to above, the coils arequite small and ideally are small enough (compared to thetransmitter-receiver distance) that they may, as an initialapproximation, be considered to exhibit dipole behavior. However, moregenerally, the system includes embodiments discussed more fully below,wherein one or more of the coils are modeled or their signals processedas finite size transmitter or receiver.

It should be noted that the closed magnetic generating or receivingcircuit includes the coil, its connecting cable and some electronics towhich the cable is connected. In accordance with a preferred embodimentof the present invention discussed further below, contributions of theseextrinsic or ancillary circuit elements are reduced or eliminated,permitting a simple and highly accurate model of the coil alone with adirect short across the coil leads.

FIG. 1 schematically illustrates a basic measurement system 1 employedin a prototype embodiment. A transmitter assembly 10 is driven by atransmitter driver 11 to produce a magnetic field, while a receiverassembly 12 responds to the magnetic field by producing induced signals,coupling the signals induced therein to a receiver voltage preamp 13 andan analog to digital converter 14. The transmitter assembly 10 andreceiver assembly 12 may, for example, each include three coils,oriented along respective spatial axes. The digitized values of thereceiver voltages from the digital converter 14 pass to a digital signalprocessing system 18 that may be implemented in or may interface with ahost processor 20 Values of the transmitter coil drive signals 14 arealso digitized by an analog to digital converter, and pass on line 15,to the digital signal processing system 18. In a preferred embodiment,the driver 11 applies known or approximately known voltages, rather thanknown or approximately known currents, to the transmitter assembly 10,and measures one or more currents induced in the coils of thetransmitter assembly. This voltage-driven approach advantageouslyalleviates the need to control the phase of one or more currents appliedto the transmitter coils, and further permits the system to functioneven with mistuned and/or non-orthogonal transmitter coils. Those havingordinary skill in the art will appreciate that the transmitter in asystem according to the teachings of the invention can also be driven bycurrents rather than voltages.

The various signals are transmitted, sampled, and combined and processedin coordination, for which operation a timing and synchronizationcontrol unit 30 forms a number of basic synchronized timing signals todefine or control, for example, the multiplexing of signals between thetransmission and receiving coils, control timing of the digitizingcircuits, control the analog-to-digital (ADC) sampling rate, and theprocessing of the digitized raw signals from the coils to produce aprocessed raw data output. The host processor also operates with the rawdata to effect the computational algorithms for determining coordinatesand correcting position and orientation determinations.

FIG. 2 shows a more detailed hardware block diagram of this aspect of atracking system which, in accordance with the present invention, isconfigured to make a robust and accurate determination of mutualinductance of the various transmitter and receiver assembly coil pairs.That information is then applied as further described below to determinethe position and orientation coordinates of one assembly with respect tothe other. For simplicity, the same numerals as employed in FIG. 1 areused to designate the transmitter, receiver, host processor, digitalsignal processor and synchronization or control circuitry.

As shown in this embodiment, a clock generator circuit 30 providestiming signals which are applied to different units for effectingsynchronized processing described further below. These units include asignal sampling analog-to-digital converter 114 a (sometimes referred toas ADC 114 or simply an A/D unit, below), which digitizes the signalvalues received from the receiver assembly and the drive signal valuessampled from the transmitter assembly; a digital-to-analog converter(DAC) 114 b which converts digital drive waveform signals to analogvoltages for powering the amplifier which supplies transmitter drivesignals; and a digital signal processing unit 18 and wave form generator40.

The wave form generator 40 is a digital frequency synthesizer thatgenerates digital values of a sine wave signal at one or more selectedfrequencies. These may include, for example, a separate frequency fordriving each of the transmitter coils. These digital values areconverted to analog values by DAC 114 b, and are fed to a poweramplifier and filter 50 for driving each transmitter coil.Advantageously, the power amplifier may be a voltage amplifier. Thedrive signal at any given time, is switched by a current multiplexer tothe appropriate coil, and simultaneously the induced voltages in thereceiving coils are demodulated at the frequency of the transmitteddrive signal. Meanwhile, the drive current to the transmitter assemblyis detected by a current sensor 51 a and is measured by currentmeasuring circuitry 60.

As further shown in FIG. 2, the receiver coil assembly includes acalibration transmitting coil 10 a fixed at a position close to thethree receiver coils of the assembly. Coil 10 a may be, for example,immovably secured in a fixed orientation relative to the three receivercoils in an integrated unit. The coil 10 a is operated to generate alocal magnetic field, oriented so that it induces calibration voltagesignals in each of the receiving coils. The drive current in this coilis also measured by the current measuring circuitry 60. Coil 10 a isenergized, as described further below in connection with determinationof mutual inductances, to effect normalization of the component signalsreceived in the three receiving coils R_(x), R_(y), R_(z). Thecalibration coil 10 a is driven by a calibration coil driver.

The receiver coils R_(x), R_(y), R_(z), provide their voltage outputs toa preamp and filter 80, the output of which is fed to theanalog-to-digital converter ADC 114 a, as described above, to producedigital signal values for processing. Converter 114 a also receives asan input, the outputs of the current measuring circuitry 60, so that theADC 114 a provides digital values of both the calibration coil currentand the transmitter coil currents to the digital signal processing unit18. Coordination between the various transmitting, receiving, sensingand converting steps is carried out with timing signals from the clockgenerator circuitry 30. By way of example, clock circuitry 30 mayoperate with a 25 megahertz clock, dividing it down (e.g., by 256) toset an ADC sample rate of 97.65625 kilohertz, which is successivelydivided by 2, by 1440, and by 256 to provide sample rate framing signalsof 48.828125 kilohertz for the DAC, and end of cycle frame signal at33.90842 Hz for a sum of products (SOP) processing module implemented inthe signal processor DSP 18. Further frequency division may provide acurrent multiplexer cycle rate (for switching transmitter drive cyclesto different coils) of 0.13245477 Hertz, or other synchronized timingsignals as appropriate. Thus, operation of the various hardware unitsshown in FIG. 2 is synchronized by the clock generator circuit 30. Suchan exemplary timing unit is shown in FIG. 5E.

One of the clock signals is fed to a digital wave form generator 40 thatsynthesizes three distinct drive frequencies for the x-, y- andz-transmitter coils. It will be understood that the digital wave formgenerator 40 may be implemented in the host processor 20. However, it ispreferably a specialized frequency synthesis chip or circuit, whichproduces a digital continuous sine wave of precise frequency and phase.As described further below, the different frequencies are used to driveindependent magnetic field generator coils, and corresponding signalsinduced in receiver coils are detected and processed in a simple digitaldemodulator, a sum-of-products processor. Preferably, the synthesizedfrequencies are integral multiples of the sum of products digital signalprocessing cycle rates (i.e., the cycle rates of the DSP unit 18) sothat the sine wave repeats exactly every SOP cycle with no discontinuitybetween cycles. The frequencies so generated are fixed, and the sinewave generator may be implemented in a known fashion by precalculatingsamples of the desired sign wave signals and storing the samples in aprogrammable read only memory (PROM). The PROM addresses are then drivenby signals from timing generating counters (not shown) so that the PROMoutputs the samples in the serial format required by thedigital-to-analog converter 114 b. Another PROM output may implement thedivide by 1440 control logic.

The digital output values of generator 40 are converted to adifferential analog voltage in the DAC circuitry 114 b, which may, forexample, be a 24-bit Deltasigma digital-analog-converter. Its outputanalog signal is amplified by the amplifier 50 which may, for example,be implemented with a low pass filter and a fixed gain differentialamplifier.

As is apparent from the circuitry described thus far, systems of theinvention set out to measure both the transmitter coil drive currentsand the receiver coil induced voltages. These are ratiometricallycombined to form mutual inductance measurements. Various furtherconstruction details may be practiced in order to have the detectedsignals represent as accurately as possible the desired positional fielddependence. FIGS. 5A-5D illustrate further construction details usefulin preferred constructions of the transmitting and receiving hardware.

As shown in FIG. 5A, each transmitting coil is driven by a differentialamplifier 50 that amplifies the analog-converted synthesized drivefrequency signal. The r input of the differential amplifier is connectedto the low end of the transmitter coil (denoted L in the Figure), whichis also connected to a virtual ground. A coaxial cable 55 carries thedrive signal to the high end of the transmitter coil L, thus avoidingintroduction of electromagnetic fields due to currents within the drivecable. The virtual ground, although not exactly a ground voltage, isconnected to the r input of the amplifier, thus ensuring that the actualvoltage applied across the transmitter coil will not be affected bysmall stray voltages on the virtual ground. In this manner, a stabledrive voltage is applied across the transmitter coil.

Further, because the impedance of the amplifier r input is high comparedto the impedance of virtual ground or real ground, and its currentsignal is low, the current drawn through the r connection issubstantially less than (e.g., typically 10⁴ times less than) thecurrent through the coil itself. This small error may be estimated andcorrected mathematically in subsequent processing. Similarly, thecoaxial cable capacitance may shunt a small current around the coil;this current may also be estimated and corrected mathematically.

As further shown in FIG. 5A, as a coil is driven by the amplifier 50,the coil current is sampled between the low end of the coil and the rterminal of the driving amplifier. The detected drive current is appliedtogether with induced receiver coil signals for the computation ofmutual inductance.

In addition, as shown in FIG. 5B, a reference impedance measurement isalso provided at the transmitter by having a defined drive signalperiodically switched through a reference impedance Z_(ref). Theimpedance of Z_(ref) is much larger than the coil to coil mutualimpedances, so the Z_(ref) driver provides a smaller current. As seen inFIG. 5B, a resistor, R_(c), which is large compared to Z_(ref), convertsthe voltage output of the difference amplifier 50′ to a small current inZ_(ref). This connection, viz., the difference amplifier input r, isgrounded, rather than connected to the low end of Z_(ref), for tworeasons. First, the input r draws a small current which is significantcompared to the current in Z_(ref). Second, the receiver measurementchannel responses to Z_(ref) are measured only when the current throughZ_(ref) is being measured by the current-measurement channel. Thereference impedance Z_(ref) may plug directly onto the connectors on thereceiver electronics board thus avoiding the introduction of artifactsor the parasitic characteristics of cabling. In this embodiment, Z_(ref)is utilized only for determining the mutual inductances L_(cal) betweenthe calibration transmitting coil 10 a (FIG. 2) and the receiver coilsat the time of manufacturing the receiver assembly. While tracking, thethree L_(cal) values corresponding to mutual inductances between thecalibration transmitting coil 10 a and each of the three receiver coilsare employed as mutual inductance standards.

The overall arrangement of the transmitter section is shown in FIG. 5C.The driver/amplifier 50 is connected in turn to each of the coils andreferenced by a current-switching multiplexer (CMX) 60. The virtualgrounds for all the transmitter coils go through a current-mode switchmatrix, that connects none or one transmitter coil current to theprimary winding (e.g., a single-turn primary) of a current transformerT_(c), while connecting all the remaining transmitter coils to thehigh-current transmitter-coil common-ground point t. The selectedcurrent thus goes through the primary of the current transformer T_(c)to the ground point t. A suitable current transformer T_(c) may have aprimary-to-secondary turns ratio of about 1:200. It operates to isolatethe large transmitter currents from analog ground of the receiverelectronics, accurately reducing the transmitter currents to valueswithin the range of the virtual-ground preamp, and minimizing thereflected virtual-ground impedance seen by the transmitter coils.

The mutual-impedance reference is preferably operated at a lower currentthan the transmitter coils are operated, so themutual-impedance-reference current is switched in or out (as shown) onthe secondary side of the current transformer T_(c). Moreover, thesecondary of T_(c) is always in the circuit, to cancel effects due totransformer magnetizing current, so that the magnetizing current thenappears as just another constant complex gain error in thecurrent-measurement channel. Such constant gain errors cancel out indata-reduction calculations. In FIG. 5C, the op amp and resistor R_(f)form a virtual-ground current-input, voltage-output amplifier.

FIG. 5D provides an arrangement alternative to that shown in FIG. 5C. Inthis alternative arrangement, three transformers (62, 64, 66) areutilized, and the current switches are connected to the secondarywindings of the transformers. The impedance variation seen by eachtransformer coil in the transformer primary circuit, as a result ofopening and closing the corresponding switch, is much lower than thevariation seen in the arrangement of FIG. 5C. In particular, thevariations are reduced by a factor equal to the square of thetransformer turns ratio. For example, the use of a turns ratio of 1:200in the arrangement of FIG. 5D results in reducing the impedancevariations by a factor of 40,000 relative to that in the arrangement ofFIG. 5C.

In operation of the system, magnetic fields generated by the transmittercoils induce voltage signals in the receiver coils, and these arereceived and sampled to develop raw received signal values for theprocessing. The voltage-input preamp and ADC channel for one receivercoil L is shown in FIG. 5E. The voltage from the inductor L is appliedvia a coaxial cable 55 to the differential-voltage-input preamp. Thepreamp output is applied to the input of an analog to digital converter,such as the above described 24-bit Deltasigma ADC, through a passivelowpass filter. The filter eliminates alias responses as well as effectsdue to nonlinear ADC input currents. Use of a coaxial cable has theadvantageous benefit that external electromagnetic fields can induce nopotentials between the center conductor and the shield of the coaxialcable (assuming a perfectly-conducting shield). By contrast, aconventional twisted-pair cable may cancel effects due to uniformapplied electromagnetic fields, but can introduce extraneous signals inthe presence of gradient fields (such as spatially varying fields) areused in the system.

Thus, in the illustrated embodiment, there is one voltage input channelfor each receiver coil, and a channel for the current preamp.

Overall operation 200 of the position sensing system proceeds as shownin FIG. 3. The transmitting x-, y- and z-coils T_(x), T_(y), T_(z) aredriven to generate magnetic fields, and the voltages induced in thethree sensor receiver coils denoted R_(x), R_(y), R_(z) are measured. Inaddition, a reference magnetic field is generated in a calibration coil10 a coil fixed in or close to the receiving coil assembly. The currentsflowing through the transmitting coils and the current flowing throughthe calibration coil are each measured, and all the signals, includingthe received voltage signals in each receiver coil, are digitized andpassed to a signal processing unit described below more fully inconnection with FIG. 4. The ADC sampling is controlled by a commontiming generator so that the timing relationship between ADC samples andDAC samples is tightly controlled. A suitable timing module is shown inFIG. 5F, having the representative framing, cycle time and otherprocessing or sampling intervals described above. The ADC outputs, andtiming reference information from the timing generator pass to a digitalsignal processing system. The signal processing applies quadraturedemodulation (i.e., digital heterodyning at the driving frequency of thecurrently-actuated transmitting x-, y-, z- or calibration coil signal)to accumulate sum-of-products (SOP) complex raw signal values, andcomputes various matrices which allow one to normalize the couplingbetween the coils and the response of the receiver coils to thecalibration coil.

These are used in the next stage to form a nine element mutualinductance matrix for all of the (3) transmitter and (3) receiver coilpairs. The matrix is then used to compute coordinates of the receivercoil assembly with respect to the transmitter. The overall approach forthe latter computation may be similar to that employed in a conventionalfield model dipole approach, such as that described in the 1979 paper ofF. H. Raab et al. Volume AES-15, No.5, pp. 709-718, and Frederick H.Raab. Quasi-static Magnetic Field Technique for Determining Position andOrientation. IEEE Transactions on Geoscience and Remote Sensing. GE-19(4): 235-243, October 1981. For computation of position and orientationcoordinates, the processing preferably proceeds by starting with aninitial estimate of the six position and orientation coordinates,applying those coordinates to compute a mutual inductance matrix, anditeratively refining the coordinate estimate to enhance the goodness offit between the mutual inductance values as measured, and thosepredicted from the estimate.

As discussed further below, various features of the processing make themutual inductance model highly accurate and not prone to errors of scaleor position. Moreover since direct solutions are possible, the iterationproceeds quickly so that the position and orientation coordinates may beoutput based on the estimated coordinates and measured mutual inductancematrix. Appendix A attached hereto illustrates a suitable calculationfor deriving an initial set of position and orientation coordinates froma measured set of dipole-modeled mutual inductances L_(tr) of thetransmitter/receiver coil pairs. The dimensions of the coils are assumedto be small compared to their separation, allowing derivation, forexample, as described in the paper of Frederick H. Raab. Quasi-staticMagnetic Field Technique for Determining Position and Orientation. IEEETransactions on Geoscience and Remote Sensing. GE-19 (4): 235-243,October 1981. Mutual inductances may be calculated for dipole coils orfor circuits comprised of straight line segments as outlined in AppendixB and Appendix C. Each of the foregoing appendices is herebyincorporated herein by reference and made a part of this description.

FIG. 4 is a block diagram showing the digital signal processing 400undertaken with the digitized coil drive current and sensing voltagemeasurements. As shown, each of the signal measurements is digitized ina high precision (24 bit) ADC. The signals are filtered andsynchronously quadrature demodulated, for example, using aDolch-Chebyshev FIR filter, and the complex signal values are thenapplied to construct three matrices 410, 420, 430 which are used todetermine the mutual inductances and to normalize measurements. Matrix410 is a raw signal matrix made of the signal components induced in eachof the receiver coils by each of the transmitter coils. Matrix 420 is a3×3 diagonal matrix, the three entries of which represent the responseof the respective receiver coils to the calibration coil. Matrix 430 isa 3×3 current matrix with entries representing the current through eachcoil (on the diagonal) and the current due to the other coils (off thediagonal). That is, it is a cross-coupling matrix. The DSP or processoralso computes the inverse 435 of this matrix, and the current throughthe calibration coil is provided as a separate parameter 440.

In general, the mutual impedance measuring system is designed formeasurement of small mutual impedances at frequencies between 100 Hz and40 kHz, and preferably between about 10 kHz and 15 kHz. The referenceimpedance Z_(ef).may be a 10 ohm±100 ppm four-terminal resistor, so thatZ_(ref) is real and independent of frequency at the frequencies ofinterest. The whole system is ratiometric: the mutual impedances aremeasured with respect to Z_(ref) so that no precision voltage or currentreferences are required. Moreover, the frequency responses of thereceiver measuring channel and the current measuring channel are thesame, so that factors of the complex gains that vary with frequency willcancel in the rationing process. Similarly, frequency-independentcomplex gain factors of the receiver measuring channels will also cancelin the determining the ratio of mutual impedances to Z_(ref).

Overall, the system operates by converting the raw signal measurementsas discussed above to a mutual inductance matrix, and then solving forposition and orientation (P&O) from the mutual inductance.

The first step to computing P&O is converting sum-of-products (SOP) datainto a normalized raw signal matrix (RSM) of mutual inductances. The SOPdata is processed to preserve phase information, so there is a real andan imaginary component of each parameter. The unnormalized matrix hasthese complex entries., while, the mutual inductance matrix consistssolely of real numbers. As discussed above, it is the end result ofnormalizing a complex signal matrix (CSM) for the effects of current,coil and calibration coil response. The goal of this step is to end upwith a 3×3 matrix containing the mutual inductances between all possiblecombinations of sensor and transmitter coils. Because there are threesensor and three transmitter coils, there are a total of nine elementsin the signal matrix. Each row of the matrix corresponds to a sensorcoil and each column to a transmitter coil.

The RSM is computed by performing a series of matrix multiplicationoperations. The effects of transmitter current are normalized out(because the B field is directly proportional to the current through thetransmitter), as are the effects of frequency (db/dt) and receiverchannel gain (using the calibration coil response matrices). The formulais shown below: RSM(the  mutual  inductance  matrix)equals${ICM}*\begin{pmatrix}{{{Fcal}/F}\quad 0} & 0 & 0 \\0 & {{{Fcal}/F}\quad 1} & 0 \\0 & 0 & {{Fcal}/{F2}}\end{pmatrix}*{CSM}*\begin{pmatrix}{{Fcal\_ sop}{{\_ C}/{Fcal\_ sop}}\_ 0} & 0 & 0 \\0 & {{Fcal\_ sop}{{\_ C}/{Fcal\_ sop}}\_ 1} & 0 \\0 & 0 & {{Fcal\_ sop}{{\_ C}/{Fcal\_ sop}}\_ 2}\end{pmatrix}*\begin{pmatrix}{{cc\_ cf}\lbrack 0\rbrack} & 0 & 0 \\0 & {{cc\_ cf}\lbrack 1\rbrack} & 0 \\0 & 0 & {{cc\_ cf}\lbrack 2\rbrack}\end{pmatrix}$

where F0, F1, F2 are the frequencies of the x-, y- and z-transmittercoils, and where the complex entry Fcal_sop_C is the measuredsum-of-products current through the calibration coil, Fcal_sop_0 is theresponse of the x-sensor coil to the calibration coil field, etc.

The above mutual inductance matrix is then applied to determine theposition and orientation coordinates of the coil assembly. Thisoperation proceeds by first obtaining a good estimate of P&O, forexample, by using the direct dipole solution of the Raab paper citedabove (which may be augmented to correct for coil size and known fielddisturbances). The processor first factors out the effects of gain &non-orthogonality of both the receiver and transmitter coils. This maybe done by pre-multiplying the RSM by the receiver gain matrix inverse,and post-multiplying by the inverse of the transmitter gain matrix.Detailed mathematical derivation of this method is given in the attachedAppendix A.

Specifically,

 R=(Recr_gain_matrix⁻¹)*RSM*(Trans_gain_matrix⁻¹)

where R is now a 3×3 matrix where the effects of receiver andtransmitter gain and non-orthogonalities have been removed. One can nowestimate the range, r, (the square root of x²+y²+z²) and the magnitudeof the x,y,z coordinates based on the square of the total field from thethree transmitter coils.

The pertinent equations (1-4) are: $\begin{matrix}{r =^{6}\sqrt{6.0*{k^{2}/{Stotal}}}} & (1) \\{x = {{r^{4}/k}*\sqrt{\left( {{5.0*{Sx}} - {Sy} - {Sz}} \right)/18}}} & (2) \\{y = {{r^{4}/k}*\sqrt{\left( {{5.0*{Sy}} - {Sx} - {Sz}} \right)/18}}} & (3) \\{z = {{r^{4}/k}*\sqrt{\left( {{5.0*{Sz}} - {Sy} - {{Sx}/18}} \right.}}} & (4)\end{matrix}$

where, k is a scale factor that is the product of the area and number ofturns of receiver and transmitter coils and permeability of free space.Stotal is the sum of signal matrix elements squared. Sx, Sy, Sz, are thesum of signal matrix elements squared for the X,Y,Z transmitter coils,respectively.

Because of the hemispherical ambiguity associated with the tracker, theprocessor may always assume it is operating in the +X hemisphere, andsimply take dot products between sensor output vectors (the rows in atransposed signal matrix) to determine the sign of y and z.

However, the solution is unstable if any of the coordinate values isclose to zero. (See Appendix A). To correct for this, the processorpreferably mathematically rotates the signal matrix elements to move thesolution into an optimal (non-zero) region which herein refers to aregion in which the numerical coordinate values are substantiallynon-vanishing such that a stable solution of the coordinate values canbe obtained by utilizing the above methodology The rotation isdetermined by computing the quaternion effective to move the initialposition estimate to an “ideal” solution location where x=y=z (i.e.,away from the x, y and z axes). This ensures that the solution isnumerically accurate and stable. The processor then recalculatesequations (1)-(4) with the rotated R matrix. After determining these“rotated” position coordinates, the processor de-rotates with theinverse quaternion to obtain a better estimate of the true position.Again, dot products are employed to determine the sign of thecoordinates.

The r, x, y and z values are then used to compute 3×3 matrices Tα, Tβand their inverses. The matrices Tα, Tβ are rotation matrices built fromthe (positional) spherical coordinates angles, α, β, respectively. Note,however, that because

cos(α)=x/sqrt(x ² +y ²)

sin(β)=z/r

Tα, Tβ and their inverses (Tα⁻¹, Tβ⁻¹) may be constructed directly fromx,y, and z. These matrices along with the inverse of a constantsensitivity matrix, S⁻¹, allow the inverse-coupling matrix, Q⁻¹(described in the aforesaid Raab paper) to be computed.

Basically,

Q ⁻¹ =Tα ⁻¹ *Tβ ⁻¹ *S ⁻¹ *Tβ*Tα  (5)

With Q⁻¹ and the signal matrix, R, the processor computes the rotationmatrix, A, that converts a zero-orientation sensor output into theoutput of the true sensor:

A=(r ³ /k)*R*Q ⁻¹  (6)

where the elements of A can be described by a rotation quaternion,q={qs,qx,qy,qz}:

A[0][0]=(qs ² +qx ² −qz ²),

 A[0][1]=2*(qy*qx+qs*qz),

A[0][2]=2*(qz*qx−qs*qy),

A[1][0]=2*(qy*qx−qs*qz),

A[1][1]=(qs ² −qx ² +qy ² −qz ²),

A[1][2]=2*(qz*qy+qs*qx),

A[2][0]=2*(qz*qx+qs*qy),

A[2][1]=2*(qz*qy−qs*qx),

A[2][2]=(qs ² −qx ² +qy ² +qz ²).

The elements of A may be used to find a rotation quaternion, ‘q’, usingthe procedure in Appendix A8 of the paper of B. Horn, Closed-formsolution of absolute orientation using unit quaternions, J. Opt. Soc.Amer. Vol 4. p 629, April, 1987. Note, however, that here A is theinverse of the rotation matrix described in that paper.

The foregoing procedure provides a very good estimate for the positionand orientation of the sensor. To refine the estimate of position andorientation, the processor next applies a fitting, such as a first-orderor least square algorithm, to create a best-fit between a set of modelestimated mutual inductances and the measured mutual inductances in theRSM. The idea is to compute a set of small correction values to apply tothe estimated P&O. The basic equation is:

E+S*(delta_(—) P _(—) O_vector)=R  (7)

where

R is the measured signal matrix, formulated as a nine element vector;

E is an estimated signal matrix computed using both the direct solutionP&O estimate and a magnetic field model based on the actual geometry ofthe coils employed in the system (where the signal matrix E is alsoformulated as a nine element vector);

S is a 9×6 sensitivity matrix which holds the partial derivatives ofsignal matrix elements with respect to six position and orientationparameters (x,y,z,qx,qy,qz); and delta_P_O vector is the 6 element deltacorrection vector (Δx, Δy, Δz, Δqx, Δqy, Δqz) that is to be solved for.

The fourth component of the delta quaternion, Δqs, may be solved for bynormalizing it to unit length.

Note that because the matrix S is not square, it cannot be inverted.However, to put it into an invertible form, one can multiply it by itstranspose (S^(T)). See, for example, W. Brognan. Modern Control Theory.Prentice-Hall, 1991, pp 222-223. The least squares equation we end upwith is:

delta_(—) P _(—) O_vector=[Δx,Δy,Δz,Δqx,Δqy,Δqz]^(T)=(S ^(T) *S)⁻¹*(S^(T))*(R-E)  (8)

Finally, sensor position is adjusted by:

x=x+Δx; y=y+Δy; z=z+Δz

and the sensor orientation (as a quaternion), qorient, is adjusted byrotating it by the computed delta quaternion, qdelta:

qorient=qdelta*qorient

Thus, the processor operating with SOP data from the magnetic trackingunits computes and refine the position and orientation of the movablecoil assembly.

As noted above, certain structures present in the tracking arena mayintroduce field distortions because eddy currents induced in conductivestructures will themselves generate magnetic fields. In accordance withother aspects of the invention, this problem is addressed in one or moreof several ways.

One of these ways is to mount the receiver (sensor) of the system on orclose to a major interfering structure, such as (in a medical context)the C arm of a fluoroscope assembly. The transmitter coil assembly isthen the movable assembly of the tracker, and will thus be generallypositioned remotely from the interfering structure. The mutualinductance tracker of the invention can be configured to operateeffectively with small coils which may be carried by a hand-held tooland the like, thereby rendering the above approach feasible. Further,the use of mutual inductance between the transmitter and receiver unitsin the electromagnetic tracker of the invention allows swapping thefunctionality of the transmitter and receiver units, e.g., utilizing anominal sensor coil as a transmitter and/or a nominal transmitter coilas a receiver. This reciprocity can be particularly advantageousbecause, in some cases, it may be more convenient to map and/or modelthe field generated by one unit, e.g., receiver, rather than the other,e.g., transmitter.

Another aspect of the invention dealing with the field disturbanceproblem is to mount a standardized conductor about such an interferingstructure to operate as a shield (e.g., with respect to disturbancesoriginating within the shield) and a known disturber, as seen by asensor positioned outside the shield. The standardized conductor, suchas a metal can, then has a fixed position relative to one of the coilassemblies, and may be effectively modeled. Thus, the eddy currentsinduced in the sheet metal cylinder by the magnetic field from thetransmitter assembly, and the secondary B field formed by these inducedcurrents, may be modeled such that the processor can apply determinablefield contributions from the shield. Without such a shield, applicanthas found that when a coil assembly is employed in proximity to certaininterfering structures (such as the imaging assembly of a fluoroscope),the disturbance caused by that structure may vary considerably for twodifferent but apparently identical units. The shield moderates suchdifferences; electromagnetic fields originating within the shield arelargely null outside the shield. Moreover, little of the tracking signalorigination outside the shield penetrates inside, so that the level ofeddy currents induced in the structural conductive elements inside theshield are subject only to quite low fields and their eddy currents andsecondary field effects are greatly diminished. These combined effectsallow a tracking system to operate effectively by applying standardized(modeled or calibrated) corrections for the shield alone, without regardto the specific structure(such a s a fluoroscope imaging assembly) thatit isolates.

Preferably, the shield is made of a single piece of a highly conductingmaterial, e.g., a metal such as silver, copper or aluminum, or aconductive composite. The shield surrounds the targeted component orpiece of interfering equipment, such as the image intensifier assemblyof a fluoroscope, as much as possible, consistent with systemconstraints (e.g., X-ray transparency of the top end, mechanicalprotrusion limits for camera movement, etc.). Preferably, the shieldthickness is sufficient, e.g., more than the skin depth of the metal ormore than several skin depths, so that the magnetic field it generateswill not appreciably penetrate the shield or extend into the volumebehind the shield.

The field distortions introduced by the shield may be mapped and atracker-correction table may be generated in a preliminary set-up orcalibration step, or the effect of the shield may be modeled.

The shield greatly reduces the magnitude of equipment-related magneticfield distortion reaching the targeted structure, and introduces itsown, highly determined effects. Thus, as applied, for example to afluoroscope, changing the image intensifier has no appreciable effect onthe resulting field distortion, and one field distortion correctiontable can be used for all image intensifiers of a given model, andsometimes even for image intensifiers of different models.

FIG. 6 illustrates such a system in an embodiment 10 for use in surgicalnavigation in an operating room environment. As shown, the system 10includes a fluoroscope 20, a work station 30 having one or more displays32, 33 and a keyboard/mouse or other user interface 34, and a pluralityof tracking elements T1, T2, T3. The displays may illustrate imagedviews or synthesized views 40′ of tool trajectories, tissue or the like.The fluoroscope 20 is illustrated as a C-arm fluoroscope in which anx-ray source 22 is mounted on a structural member or C-arm 26 oppositeto an x-ray receiving and detecting unit, referred to herein as animaging assembly 24. The C-arm moves about a patient for producing twodimensional projection images of the patient from different angles. Thepatient remains positioned between the source and the imaging assembly24, and may, for example, be situated on a table 14 or other support.The patient may be secured in a fixed position on the support and thesupport may be tracked, or more typically, the patient may be movablypositioned. In the latter case, tracking elements T3 may be affixed toone or more relevant portions of the patient's body. In the illustratedsystem, tracking elements are mounted such that one element T1 isaffixed to, incorporated in or otherwise secured against movement withrespect to a surgical tool or probe 40. A second tracking unit T2 isfixed on or in relation to the fluoroscope 20, and a third tracking unitT3 is shown fixed on or in relation to the patient. The surgical toolmay be a rigid probe as shown in FIG. 6, allowing the tracker T1 to befixed at any known or convenient position, such as on its handle, or thetool may be a flexible tool, such as a catheter, flexible endoscope oran articulated tool. In the latter cases, the tracker T1 is preferably asmall, localized element positioned in or at the operative tip of thetool as shown by catheter tracker T1′ in FIG. 6, to track coordinates ofthe tip within the body of the patient.

As will be understood by those having ordinary skill in the art,fluoroscopes typically operate with the x-ray source 22 positionedopposite the camera or image sensing assembly 24. While in some systems,the X-ray source is fixed overhead, and the camera is located below apatient support, the discussion below will be illustrated with regard tothe more complex case of a typical C-arm fluoroscope, in which thesource and camera are connected by a structural member, the C-arm 26,that allows movement of the source and camera assembly about the patientso it may be positioned to produce x-ray views from different angles orperspectives. In these devices, the imaging beam generally diverges atan angle, the relative locations and orientations of the source andcamera vary with position due to structural flexing and mechanicallooseness, and the position of both the source and the camera withrespect to the patient and/or a tool which it is desired to track mayall vary in different shots. Thus, the use of one or more trackingelements T3 (transmitters or receivers) on the structure itself may beuseful to quantify these movements in the surgical navigation system.Further details of this complex imaging environment may be found incommonly owned U.S. patent application Ser. Nos. 09/560,940 and09/560,608, both filed on Apr. 28, 2000, and each of which isincorporated herein by reference. Each of those patent applicationsdiscloses a tracking system wherein tracking of the fluoroscope as wellas the tool and patient produce image sets and surgical navigation ofenhanced accuracy.

The imaging beam illustrated by B in FIG. 6 diverges from the source 22in a generally truncated conical beam shape, and the C-arm 26 is movablealong a generally arcuate path to position the source and camera forimaging from different directions. This generally involves positioningthe assembly 24 as close as possible behind the relevant tissue oroperating area of the patient, while the C-arm assembly is moved roughlyabout a targeted imaging center to the desired viewing angle. The C-armor other beam structure 26 may move eccentrically or translationally toallow better clearance of the patient support table, but generally, thecamera assembly 24 is positioned close to site of surgical interest.Because, as noted above, the camera 24 may utilize an image sensing unitthat introduces distortions into electromagnetic fields employed fortracking, this complicates the task of tracking, particularly in view ofthe proximity of the disturber to the tracking sensors positioned on thepatient or the surgical tool.

As further shown in FIG. 6, this problem is advantageously addressed byproviding a conductive shield/distorter 25 surrounding the imageassembly 24, which may be a metal or conductive cylinder. Preferably thetop face is open, so as to avoid degrading the X-ray image or contrastdetected by the assembly 24. Most preferably, the electromagnetictracking element T3 is a sensor element, rather than a transmitter, andit is mounted in a fixed position in relation to the shield 25. That is,the tracking element T3 is clamped to the shield 25, or clamped to theC-arm (as illustrated) or to the imaging assembly close to the shield.This device architecture is advantageously used in connection with aprocessor to achieve enhanced speed and robustness of the trackingcoordinates.

As discussed above, in a tracker of the invention, a shield may bemodeled in the processor, so that a more accurate model of the magneticfield data is present initially, either obviating or reducing the taskof actually mapping the disturbed field measurements. When the processordetermines position and orientation components by refining an iterativeestimation or fitting procedure, for example wherein the systemestimates the coordinates of a transmitter or sensor unit, or therelative coordinates by comparing calculated signal measurements withmeasured signal values, such modeling may produce faster and lesscomputationally intensive convergence of the fitting procedure. Thus,for example, a cylindrical “can” fitted about the imaging assembly of afluoroscope may be effectively modeled as a conductive ring or annulusat the defined position. This may produce a kernel or an initial seedsolution without resorting to massive stored field calibration tables,or may yield a fast or even closed form calculation of the fielddistribution for P&O determination. A traditional approach to the designof electromagnetic trackers set forth, for example, in the 1981 paper ofF. H. Raab, supra, assumes magnetic dipole field distributions for boththe transmitting and sensing coils. This assumption permits an analyticsolution for position and orientation coordinates (hereinafter, simply“P&O”) based on the measured sensor data. However, in cases where theactual fields deviate from the dipole form, application of adipole-based algorithm produces distorted P&O estimates. Such deviationsoccur, for instance, when coil sizes are significant relative to thesource-sensor separation distance, or when conducting or magneticmaterials are located near the sensors. (The term sensor is used hereinto refer to both transmitting and receiving coils unless specificallynoted). An analytic solution for P&O is not generally possible fornon-dipole field models, so in order to continue to use this formalismin the presence of real fields, a typical approach has been to correctthe P&O estimates from the dipole algorithm using a large lookup tablecontaining measured values of the distortion over the working volume, asdescribed in Zachman, G. Distortion Correction of Magnetic Fields forPosition Tracking. Proc. Computer Graphics Int'l (CGI'97), Jun. 23-27,1997, Hasselt & Diepenbeek, Belgium.

However, in accordance with another aspect of the present invention,rather than proceeding by creating a distortion map for everyapplication, a system implements a P&O determination based on a moreaccurate model of the sensor field (for example, modeling the extrinsicdistortion) that eliminates the need for a map by curing the distortionproblem at its source.

Numerical models (e.g., finite element methods) can be made arbitrarilyaccurate, but the computational cost for this accuracy may become quitehigh. As an alternative, a given source may be approximated as acollection of simple shapes for which the fields can be expressedanalytically. This approach provides a compromise between accuracy andspeed, where the accuracy will be limited by the ability to match theactual geometry with a set of simple shapes, which determine the “basicfunctions” for the expansion.

This approach has been applied in a further aspect of the invention to afluoroscopic tracking system where the sensor is mounted close to alarge parasitic element, namely the C-arm of a fluoroscope. In thiscase, a distorter element, for example, in the form of a large metal canis preferably mounted on the C-arm image intensifier to dominate thefield distortion. The sensor fields are then modeled as a three-axisdipole located near a filamentary conducting ring. The fields of both adipole and filamentary ring can be computed analytically (see, forexample, Smythe, WR. Static and Dynamic Electricity. HemispherePublishing, New York, 1989. A composite model of the coupling matrixcomponents can therefore be written as the sum of analytic expressions.The following sections describe the model in detail.

To embed the new field model in the tracker fit routine, one needs a wayto compute the coupling matrix between the transmitter and receiver inthe presence of the distortion source. To accomplish this, applicantmakes two assumptions:

(i) Assume that the transmitting and receiving sensor coils can bemodeled as magnetic dipole elements. This assumption allows for aconvenient dot product relationship between the field vector and inducedvoltages, as the coils are assumed to be point sources/receivers.

(ii) Assume that the dominant distorter is the metal can around theimage intensifier, and that we can model that can as a simple metalring. This assumption allows us to use analytic expressions for themagnetic fields radiated by the ring, which are available in theliterature (Smythe, supra or Jackson, J D Classical Electrodynamics.Wiley, New York, 1975).

Using these assumptions, the development proceeds as follows. First, usenetwork theory to derive an expression for the coupling between thetransmitter and receiver in the presence of the ring. This expressioncontains several mutual and self-inductance terms that we need tocompute. Known expressions for the coupling between two dipole elementsare used together with the fields generated by a ring to compute therequired inductances. Finally, the signal matrix elements can becomputed as a function of P&O.

The voltage induced on a receive coil due to a current flowing on atransmit coil in the presence of a metal-ring can be computed byconsidering the system as a three-port network, as schematically shownin FIG. 7.

The impedance parameters describing the coupling among the ports aregiven by: ${\begin{bmatrix}V_{1} \\V_{2} \\V_{3}\end{bmatrix} = {\begin{bmatrix}Z_{11} & Z_{12} & Z_{13} \\Z_{21} & Z_{22} & Z_{23} \\Z_{31} & Z_{32} & Z_{33}\end{bmatrix} \cdot \begin{bmatrix}I_{1} \\I_{2} \\I_{3}\end{bmatrix}}},{{{where}\quad Z_{ij}} = {\frac{V_{i}}{I_{j}}_{I_{k \neq j} = 0}}}$

(See, e.g., Pozar, D M. Microwave Engineering. Addison-Wesley, Reading,Mass., 1990.)

Consider the case where we want to know the voltage induced on port 2(the receiver) when port 1 (the transmitter) is driven with a fixedvoltage, port 2 is loaded with an impedance Of Z_(L), and port 3 (thering) is loaded with a short circuit. After some algebra, we can writethe induced voltage on port 2 relative to the current through port 1 as:$\begin{matrix}{\frac{V_{2}}{I_{1}} = {\left\lbrack {{j\quad \omega \quad L_{12}} - \frac{\left( {j\quad \omega \quad L_{13}} \right)\left( {j\quad \omega \quad L_{23}} \right)}{Z_{33}}} \right\rbrack \cdot {\left\lbrack {1 + \frac{\left( {j\quad \omega \quad L_{23}} \right)^{2}}{Z_{33} \cdot Z_{L}} - \frac{Z_{22}}{Z_{L}}} \right\rbrack^{- 1}.}}} & \left( {{Eq}.\quad 5} \right)\end{matrix}$

If we assume that Z_(L) is “large” relative to the other terms in thesecond set of brackets, and that the resistance of the ring isnegligible, Equation 5 reduces to:$\frac{V_{2}}{I_{1}} \cong {{{j\omega}\quad L_{12}} - \frac{\left( {j\quad \omega \quad L_{13}} \right)\left( {j\quad \omega \quad L_{23}} \right)}{L_{33}}}$

This approximation is sufficient for many cases, and the full expressionin (5) can be implemented where the additional accuracy is required.

To calculate the mutual inductance between the transmitting or receivingcoils and the ring, we apply reciprocity and let the ring act as thesource for both cases. By first calculating the field radiated by acurrent flowing on the ring, the voltage induced at the transmit orreceive coil is given by a simple dot product. Under the magneto-staticapproximation, the B-field produced by a uniform current I flowing on afilamentary ring (FIG. 8) is given in cylindrical coordinates by Eq.(7):${B_{\rho} = {\frac{\mu \cdot I}{2\pi} \cdot \frac{z}{\rho \cdot \left\lbrack {\left( {a + \rho} \right)^{2} + z^{2}} \right\rbrack^{1/2}} \cdot \left\lbrack {{- K} + {\frac{a^{2} + \rho^{2} + z^{2}}{\left( {a - \rho} \right)^{2} + z^{2}} \cdot E}} \right\rbrack}},{B_{z} = {\frac{\mu \cdot I}{2\pi} \cdot \frac{1}{\left\lbrack {\left( {a + \rho} \right)^{2} + z^{2}} \right\rbrack^{1/2}} \cdot \left\lbrack {{- K} + {\frac{a^{2} - \rho^{2} - z^{2}}{\left( {a - \rho} \right)^{2} + z^{2}} \cdot E}} \right\rbrack}},{B_{\varphi} = 0},$

where K and E are complete elliptic integrals of the first and secondkind, respectively, and the argument k for the integrals is given by

 4aρ·[(a+ρ)^(2+z) ²]⁻¹.

Equation 5 requires the computation of one self inductance term andthree mutual inductance terms (additional terms must be calculated forEquation 6, which makes no approximations).

The self-inductance term is for the ring, and is given by [7]${L_{ring} = {a \cdot \left\lbrack {{\mu \cdot \left( {{\ln \frac{8a}{b}} - 2} \right)} + {\frac{1}{4}\mu^{\prime}}} \right\rbrack}},$

where

a is the radius of the loop,

b is the radius of the wire,

μ′ is the permeability of the wire material, and

μ is the permeability of free space.

The mutual inductance L₁₂ between the transmitting and receiving coilsis derived in [10] using the dipole assumption, and can be calculated as$L_{12} = {\frac{\mu_{0}A_{1}A_{2}}{4{\pi \cdot R^{3}}} \cdot \left\lbrack {{3 \cdot \left( {{\hat{A}}_{1} \cdot \hat{R}} \right) \cdot \left( {\hat{R} \cdot {\hat{A}}_{2}} \right)} - \left( {{\hat{A}}_{1} \cdot {\hat{A}}_{2}} \right)} \right\rbrack}$

where

{overscore (A)}₁=effective area of the transmitting dipole,

Â₁=unit vector in the direction of {overscore (A)}₁,

A₁=magnitude of {overscore (A)}₁,

{overscore (A)}₂=effective area of the receiving dipole,

A₂=unit vector in the direction of {overscore (A)}₂,

Â₂=magnitude of {overscore (A)}₂.

{right arrow over (R)}=the vector from the transmitter to the receiver

{circumflex over (R)}=the unit vector in the direction of {overscore(R)}

R=the magnitude of {overscore (R)}.

Given a position and orientation of the sensor relative to thetransmitter, the range R can be calculated. The effective area terms arecalculated based on the coil geometry and a calibration procedure [11].

The remaining two mutual inductance terms, those between thetransmitting coil and the ring, and between the receiving coil and thering, may be calculated in a similar manner.

The position and orientation of the ring relative to the sensor coils isassumed to be known and fixed. Therefore, to compute the mutualinductance L₂₃ in Equation 6, first compute the P&O of the sensor in thecoordinate frame of the ring shown in FIG. 8. Next, compute the magneticfield per unit current produced by the ring at the sensor location.Finally, the mutual inductance term is calculated for each sensor coilby taking the dot product between the coil vector and the field, usingthe point source property of the dipole approximation.

Computing the mutual inductance between the transmitter and the ringrequires one additional step. Given the P&O of the sensor in thetransmitter coordinate system, we first convert so that we have the P&Oof the transmitter in the sensor coordinate frame. The procedure thenproceeds as above. The mutual inductance L₁₃ is computed by calculatingthe P&O of the transmitter in the ring coordinate system. Therelationship between the ring and the sensor is known and fixed as notedabove, so once the transmitter P&O is known relative to the sensor,conversion transformations are obvious. Next compute the magnetic fieldper unit current produced by the ring at the sensor location. Finally,the mutual inductance term is calculated for each sensor coil by takingthe dot product between the coil vector and the field, using the pointsource property of the dipole approximation.

With all of the inductance terms available, Equation 6 is then readilycalculated. Overall, the approach of measuring mutual inductances allowsthe system to develop robust measurements in which reliance onreferences such as Z_(ref) or a signal ratio, or use of a commonpreamplifier with high precision output digitization, have eliminatednumerous sources of variability while allowing effective use of smallmagnetic assemblies to achieve accuracy and resolution.

Moreover, by introducing a shield or virtual distorter and fixing areceiver assembly with respect to the shield, the effects of individualdistortion environments are substantially eliminated, and the shielditself may be effectively modeled, eliminating the cumbersomerequirement of compiling a mapping or calibration table. The model maythen produce a seed calculation, allowing computationally effectivefitting processes to replace the cumbersome calculations of the priorart. In other words, a distorter that is shaped to optimally shieldmagnetic fields from existing environmental distorters can be utilizedwithout any regard for a specific desired field shape. The shield'seffect on the field can then be modeled, or alternatively, mapped.

The invention being thus disclosed and illustrative embodiments depictedherein, further variations and modifications of the invention will occurto those skilled in the art, and all such variations and modificationsare considered to be within the scope of the invention, as defined bythe claims appended hereto and equivalents thereof.

What is claimed is:
 1. An electromagnetic tracking system comprising amagnetic field generating unit driven by a drive signal, a field sensingunit having a sensing signal responsive to a changing magnetic field,the generating and sensing units being arranged to generate and tosense, respectively, an electromagnetic field in an arena of interest,and wherein at least one of said units is movable, signal measurementand conditioning circuitry connected to said units to (i) detect thedrive signal and the sensing signal, (ii) ratiometrically combine saidsignals to form a mutual inductance matrix, and a processor operativewith the mutual inductance matrix to determine coordinates of themovable unit.
 2. The electromagnetic tracking system of claim 1, whereinsaid system drives the field generating unit with a controlled voltagedrive line, and senses current in said line.
 3. The electromagnetictracking system of claim 1, wherein said field generating unit comprisesa plurality of independent field coils, a multiplexer coupled to saidcoils for selecting any one of said coils, and an amplifier coupled tosaid coils via the multiplexer, the amplifier measuring a currentflowing through any of the coils selected by the multiplexer.
 4. Theelectromagnetic tracking system of claim 1, further comprising acalibration transmitter, the calibration transmitter being fixed withrespect to the field sensing unit for generating a fixed reference fieldsensed by said sensing unit.
 5. The electromagnetic tracking system ofclaim 4, wherein the field sensing unit comprises n≧2 coils spanning n≧2dimensions, and the calibration coil is positioned to couple signal intoeach of said coils.
 6. The electromagnetic tracking system of claim 4,further comprising a current sensor that senses one or more currentinduced in the generating unit by the drive signal and another currentsensor senses current in the calibration transmitter, both said currentsensors being coupled to a common amplifier thereby providing a commongain factor for all current measurements.
 7. An electromagnetic trackingsystem comprising: a magnetic field generating unit configured togenerate a magnetic field signal; a magnetic field sensor responsive toa changing magnetic field, wherein at least one of said magnetic fieldgenerating unit and said magnetic field sensor is movable; signalmeasurement and control circuitry connected to said magnetic fieldgenerating unit and said magnetic field sensor, said signal measurementand control circuitry being adapted to (i) detect the generated magneticfield signal, (ii) detect a magnetic field sensor response signal outputby said magnetic field sensor, and (iii) ratiometrically combine thegenerated magnetic field signal and the magnetic field response signalto form a mutual inductance matrix; and a processor adapted to determinea position of at least one of said magnetic field generating unit andsaid magnetic field sensor through the mutual inductance matrix.
 8. Anelectromagnetic tracking system comprising a movable magnetic fieldgenerating unit driven by a first signal, a movable field sensing unitoutputting a second signal responsive to a changing magnetic field;signal measurement and conditioning circuitry connected to said units to(i) detect the first and second signals, (ii) ratiometrically combinesaid signals to form a mutual inductance matrix, and a processoroperative with the mutual inductance matrix to determine coordinates ofsaid units.